Conventionally, as a form of a hybrid vehicle including an electric motor and an internal combustion engine other than a serial form and a parallel form, as disclosed in U.S. Pat. Nos. 3,050,125, 3,050,138, 3,050,141, 3,097,572, and the like, there is a form in which the torque of the power of the internal combustion engine is converted by dividing the power of the internal combustion engine to a power generator and a driving shaft using one planetary gear mechanism (a differential gear mechanism having three rotating components) and two electric motors and driving an electric motor arranged at the driving shaft by using electric power generated by the power generator. This will be referred to as a “three-axis type”.
According to this conventional technology, the engine operating point of the internal combustion engine can be set to an arbitrary point including stop, and accordingly, the fuel efficiency can be improved. However, although not as much as for the serial form, since an electric motor having relatively sufficiently high torque is necessary for acquiring sufficient driving-shaft torque, and the amount of transmission and reception of electric power between the power generator and the electric motor increases in a low gear ratio region, the electric loss increases, and there is still room for improvement.
As methods for solving this point, there are methods disclosed in U.S. Pat. No. 3,578,451 and Japanese Patent Application Publication Laid-Open (JP-A) No. 2004-15982, and JP-A Nos. 2002-281607 and 2008-12992 applied by the applicants of the present invention.
In the method disclosed in JP-A No. 2002-281607, a driving shaft connected to an output shaft of an internal combustion engine, a first motor generator (hereinafter, referred to as “MG1”), a second motor generator (hereinafter, referred to as “MG2”), and a drive wheel is connected to each rotating component of a differential gear mechanism having four rotating components, the power of the internal combustion engine and the power of the MG1 and MG2 are combined, and the combined power is output to the driving shaft.
In addition, in the method disclosed in JP-A No. 2002-281607, by arranging an output shaft of an internal combustion engine and a drive shaft connected to a drive wheel in a rotating component arranged on the inner side on an alignment chart and arranging the MG1 (the internal combustion engine side) and MG2 (the driving shaft side) in a rotating component disposed on the outer side on the alignment chart, the ratio of power that is in charge of the MG1 and MG2 to the power delivered to the driving shaft from the internal combustion engine can decrease, whereby the MG1 and MG2 can be miniaturized, and the transmission efficiency of the driving device can be improved. This will be referred to as a “four-axis type”.
In addition, a method disclosed in U.S. Pat. No. 3,578,451 similar to the above-described method has been proposed, in which an additional fifth rotating component is included, and a brake stopping this rotating component is arranged.
In JP-A No. 2008-12992, in a drive control device of a hybrid vehicle including an internal combustion engine and a plurality of motor generators, a technology for controlling the internal combustion engine has been disclosed in which the engine rotation speed is set high at the engine operating point of the internal combustion engine.
In the above-described conventional technology, as disclosed in U.S. Pat. No. 3,050,125, the power to be output by the internal combustion engine is calculated by adding the driving force required for the vehicle and the electric power required for charging a battery, and, out of a combination of the engine torque and the engine rotation speed corresponding to the power, a point at which the efficiency is high as possibly as can is calculated and is set as a target engine operating point. Then, the engine rotation speed is controlled by controlling the MG1 such that the engine operating point of the internal combustion engine becomes the target operating point.